Various magnetic beads useful for depletion of targeted cells are commercially available [see, e.g., U.S. Pat. No. 5,466,609]. However, metallic particles in magnetic applications [M. P. Sharrock, "Particulate Recording Media", in MRS Bulletin (March 1990), pp. 53-61; R. M. White, Science, 229:11-15 (1985)] display an undesirable tendency towards oxidation or other reactions caused by the elemental state of the metal combined with the high specific surface area common to all small particles. Thus, presently the magnetic beads with superior performance in immunomagnetic separations are DYNABEAD M-450.RTM. 4.5 micron diameter beads (DYNAL, Inc., Great Neck, N.Y.) which consist of monosized polystyrene particles embedded with ferrofluid .gamma.-Fe.sub.2 O.sub.3 particles. These magnetic beads can be effectively used at a 3-10 bead-to-cell ratio in positive cell selection and at a 10-40 bead-to-cell ratio in negative cell selection.
One type of metallic bead which is commonly employed is elemental nickel. Such nickel particles exhibit somewhat different ferromagnetic properties depending upon their size, either above or below about 20 nm diameter [C. Kittel and J. K. Gait, "Ferromagnetic Domain Theory", in Solid State Physics, F. Seitz and D. Turnbull, eds., Academic Press, New York, N.Y. (1956), Vol. 3, pp. 437-564]. Typical Raney (porous) nickel particles are produced on an industrial scale as a hydrogenation catalyst by thermal decomposition of nickel tetracarbonyl, and have an average diameter of 100 to 200 microns. Smaller particles in the 1 to 10 micron diameter range are also available.
Several methods have been employed to overcome the disadvantage of oxide formation as mentioned above. For example, nickel oxide layers on nickel particles can be removed by heat treatment. However, in order for decomposition of nickel oxide to its constituent elements in their standard states to be thermodynamically favorable, i.e., to have a negative standard free energy change, the temperature must be raised above 2540.degree. K or 2270.degree. C., otherwise nickel oxide remains unchanged. This can be shown with standard entropy and enthalpy data [S.degree. (Ni, s)=29.9 JK.sup.-1 mol.sup.-1, S.degree. (O.sub.2, g)=205.0 JK.sup.-1 mol.sup.-1, S.degree. (NiO, s)=38.0 JK.sup.-1 mol.sup.-1, .DELTA.H.sub.f .degree. (NiO, s)=-239.7 kJmol.sup.-1, .DELTA.H.sub.f .degree. (Ni, s)=.DELTA.H.sub.f .degree. (O.sub.2, g)=0] from CRC HANDBOOK OF CHEMISTRY AND PHYSICS, 64th ed., 1983-1984, CRC Press, Inc., Boca Raton, Fla., pp. D-50-D-93, which give .DELTA.S.degree. (reaction)=29.9+1/2(205.0)-38.0=94.4 JK.sup.-1 mol.sup.-1 or 0.0944 kJK.sup.-1 mol.sup.-1 and .DELTA.H.degree. (reaction)=0.0+0.0-(-239.7)=239.7 kJmol.sup.-1. Then, using the Gibbs-Helmholtz equation, .DELTA.G.degree. (reaction)=.DELTA.H.degree. (reaction)-T.DELTA.S.degree. (reaction), to find the temperature at which the standard free energy change reverses sign, .DELTA.G.degree. (reaction)=0=239.7-T(0.0944), and T=239.7/0.0944=2540 K.
Alternatively, such metallic particles can be stabilized by using alloying elements and additives, by protective organic coating, and by controlled oxidation of their surfaces, i.e., passivation. For example, antibody has been adsorbed onto nickel beads and then fixed thereon by crosslinking it with glutaraldehyde [U.S. Pat. No. 5,576,185, issued Nov. 19, 1996]. Attempts were made to enhance the nickel oxide layer with passivation by heat treatment at 250.degree. C. to sterilize the beads and create more surface area for adsorption of antibody. An oxide coating on elemental solid nickel has long been used as a protective measure to prevent corrosion. Heating the nickel beads reduced the solubilization of nickel ions from the particles and did not alter the depletion performance of the antibody-coated beads.
Nevertheless, a high level of leaching, about 100 mg nickel/mL of 25% w/v solids nickel beads after 16 weeks storage at 5.degree. C., of nickel ions from antibody-coated nickel beads suspended in bovine serum albumin (BSA) buffer still took place. Nickel oxide surfaces constantly exposed to buffered aqueous solution, unlike nickel surfaces exposed to air, are not immune to corrosion.
Thus, there exists a need in the art for compositions and methods which enable more efficient production and use of magnetic reagents for biological assays.